This invention generally relates to fully electronic ignition systems for internal combustion engines, and more particularly to means for obtaining a constant dwell time in such systems.
Conventional ignition systems store energy in the ignition coil which, upon ignition firing, creates a voltage suitable for firing a spark plug. Between times of firing, energy must be restored to the coil. Here a compromise must be made. If energy is pumped into the coil at all times except firing, power consumption is high. If, however, insufficient energy is restored, the spark energy decreases causing inefficiency in engine operation.
The problem is a function of engine RPM. At low RPM there is a relatively long time between firing, conversely, at high RPM, the coil must be recharged quickly to prepare for the next firing.
Conventional distributors use breaker points to determine the amount of time the coil is recharged. The points are simply a pair of electrical contacts -- one of which is stationary and the other is in a position to ride on a cam driven off of the engine. When the points are closed the coil is energized. The duration of time that the points are closed is defined as dwell time. Similarly, the angle on the cam through which the points are closed is defined as dwell angle. Conventional systems fix the dwell angle and, in so doing, do not adequately satisfy the tradeoff between coil efficiency and engine efficiency, there being a rapid falloff in the energy level obtained in the coil as RPM increases. Ideally, the ignition energy level should remain constant over the active range of engine RPM. The only variable in the equation for ignition energy which is conveniently alterable is the time during which energy is being restored to the coil. Thus, the way to maintain constant energy is to vary dwell angle to provide a constant dwell time.